1. Field of the Invention
The present invention relates to an image processing apparatus that converts an input chrominance signal to an image recording signal for an image formation device.
2. Description of the Related Art
Recently, as a scanner and a digital camera are popularized in addition to the development of a computer, network service and the development of a mass storage medium, a picture signal is rapidly digitized and a demand for printing such a digital picture signal with high quality rises. To acquire the printout of high quality based upon a digital picture signal, it is important to output an image with color corresponding to color information of an input picture signal. However, the color reproducibility is different every image formation device utilized in printout and when an input digital picture signal is output as it is, faithful color reproduction cannot be expected.
Therefore, an image formation device performs so-called color conversion that an input chrominance signal is converted to an image recording signal according to the color reproduction characteristic of the image formation device so that color faithful to the chromatic information of an input picture signal is reproduced. For the color conversion, a method of using matrix operation, a neural network and a multidimensional look-up table (hereinafter called DLUT) is generally used widely.
For the method of using matrix operation, relation between an input value and an output value is described in a one-dimensional or a high-dimensional determinant and the output value is acquired by operating the determinant based upon the input value. The coefficient of the determinant can be stored in a read only memory (ROM) or a random access memory (RAM) beforehand. The method of using matrix operation has an advantage that only very few parameters are required for color conversion and the capacity of ROM and RAM may be small. However, there is a problem that in case the method of using matrix operation has input-output characteristics high in nonlinearity, it is difficult to acquire the high color conversion precision.
For the method of using the neural network, relation between an input value and an output value is described in a form of a neural network and the output value corresponding to the input value is acquired. The method of using the neural network has an advantage that only relatively few parameters are required for color conversion and in case of input-output characteristics high in nonlinearity, high color conversion precision can be also acquired. However, the method of using the neural network has a problem that as the amount of operation is much, it is not suitable for real time processing.
For the method of using DLUT, an output value is stored at an address of ROM or RAM corresponding to an input signal. The method of using DLUT has an advantage that color conversion can be made at extremely high speed because an output value can be acquired by only applying an input signal as an address and operation time is not substantially required. There is also a case that interpolation is used, however, even in that case, an output value can be acquired by simple operation at high speed. Further, the method of using DLUT has another advantage that as input and output can be freely set in principle, high color conversion precision can be also acquired in case input-output characteristics have high nonlinearity.
As nonlinearity between an input chrominance signal and an image recording signal is generally very high in an image formation device, DLUT which enables high speed processing and the color conversion precision of which is high is widely utilized. However, in an image formation device the in-plane uniformity of which is low, even if color conversion is made using DLUT, it is difficult to realize high color conversion precision. The image formation device the in-plane uniformity of which is low means an image formation device wherein even if a fixed image recording signal is input, color is different depending upon a position on the recording surface of paper. For example, in an image formation device according to an ink-jet recording method and an image formation device according to electrophotography, gray of middle density is output on the whole surface of paper of size A3, the average value of colorimetric values in the plane and color difference between each point of measurement are calculated and the in-plane uniformity is evaluated by the average value and the maximum value of the color difference. For the result of the evaluation, the average value and the maximum value of the color difference are both 1 or less in the ink-jet recording method, while in electrophotography, the color difference the average value of which is approximately 3 to 5 and the maximum value of which is approximately 6 to 10 exists.
When color conversion is actually made using DLUT and color conversion precision is evaluated using color difference between an input chrominance signal and an output colorimetric value, such enough precision as the average value and the maximum value of the color difference are both 1 or less is acquired in the ink-jet recording method the in-plane uniformity of which is high, while in electrophotography the in-plane uniformity of which is low, color difference the average value of which is approximately 3 to 5 and the maximum value of which is approximately 6 to 10 exists. Therefore, in electrophotography, sufficiently high color conversion precision is not acquired. When a color conversion parameter of DLUT is determined, required color of colors in an overall range in which color is to be reproduced in an image formation device is output and a value measured by using a colorimeter is set as the color conversion parameter of DLUT. However, as colorimetric color has an error in an image formation device the in-plane uniformity of which is low, color conversion precision exceeding the in-plane uniformity of the image formation device cannot be acquired in principle. Therefore, in electrophotography, even if color conversion is made by using DLUT generated based upon correspondence with an input chrominance signal and an output colorimetric value, enough precision cannot be acquired.
It is normally said that in case color difference is 1 or less, it is not perceived even if colors are arranged, however, in case color difference is 5 or more, it is perceived without arranging colors. At the above-mentioned color conversion precision acquired in electrophotography, color difference between an input chrominance signal and the color of a printed image can be easily perceived.
Further, there is a problem that when DLUT is applied to color conversion in an image formation device the in-plane uniformity of which is low and a color conversion parameter is determined in consideration of color conversion precision, the color conversion parameter has a random error and the continuity of a color conversion characteristic is deteriorated. When the continuity of the color conversion characteristic is deteriorated as described above, a pseudocontour is formed in an image in case the gradation image is output.
To improve these problems, technique for reducing an error of a colorimetric value caused in an image formation device the in-plane uniformity of which is low is disclosed in JP-A-11-261831 for example. That is, a first color chart having plural color patches of combinations of output values of three colors varied, respectively, and a second color chart having an overall uniform output value are output, and these plural color patches and the color of the uniform output image are measured. Hereby, as an in-plane color variation value is acquired based upon the colorimetric value of the uniform output value image, color variation proper to an image formation device included in the plural color patches is removed based upon the color variation value and three-dimensional DLUT acquired by removing the color variation from these plural color patches is generated.
As the effect of an error by in-plane color variation of an image formation device can be reduced in determining a color conversion parameter value of DLUT according to this method, color conversion precision is enhanced and the formation of a pseudocontour can be reduced. However, according to this method, the effect of an error by color variation in the plane cannot be removed in principle. That is, for the color conversion parameter value of DLUT, an error by in-plane color variation is considered, however, an error by in-plane color variation when recording is actually performed based upon an output chrominance signal to which color conversion is applied by using DLUT occurs. Therefore, there is a problem of color conversion precision that an error equivalent to color variation is left.
FIG. 10 is an explanatory drawing for explaining a problem in one example of a related art. The above-mentioned problem will be described below, referring to FIG. 10. Supposing that in FIG. 10, they-axis and the x-axis respectively show lightness L* and a position on paper and color variation exists on the paper as shown in FIG. 10. In case that a color conversion parameter is determined in a position A on the paper, color difference in the position A is zero by a method of using a measured value for a color conversion parameter as it is as normally performed and color difference in a position B is 2ΔL. In case that the color conversion parameter is determined by using the method disclosed in JP-A-11-261831, since a value acquired by correcting a measured value in the position A with the average value Lave of lightness in the position A and the position B becomes a color conversion parameter, color differences in the position A and the position B are both ΔL. Therefore, average color difference by the method of using the measured value for the color conversion parameter as it is is (0+2ΔL)/2=ΔL and maximum color difference is 2ΔL. In the method disclosed in JP-A-11-261831, average color difference is (ΔL+ΔL)/2=ΔL, maximum color difference is ΔL. Though the method produces effect in reducing the maximum color difference, the effect of reducing the average color difference cannot be expected.
Therefore, in the method disclosed in JP-A-11-261831, in case that a uniform image is output in a plane, color variation caused by the in-plane uniformity of an image formation device is output without correction as it is. Therefore, in an image formation device the in-plane uniformity of which is low such as an electrophotographic printer, color variation in an output image is perceived as irregular color. According to the above described reasons, it is difficult to say that high-precision color conversion can be realized by the method disclosed in JP-A-11-261831.
As described above, in color conversion according to the related art, even if high-precision color conversion represented by DLUT is applied, high color conversion precision cannot be acquired in an image formation device the in-plane uniformity of which is low.
For an image formation method the in-plane uniformity of which is low, an electrophotographic image formation device is representative as described above. For trial to enhance the in-plane uniformity of electrophotography, some methods have been heretofore proposed.
Cause in which the in-plane uniformity of electrophotography is low can be roughly classified into two. One is color variation caused in a plane due to so-called misregistration that an image recorded position is off a desired position due to vibration of a driving system and the mechanical precision of the driving system when an image is recorded on an image carrier such as a photo conductor and a transfer belt. Another is color variation caused in a plane due to an image formation process including electrification, exposure, developing and transfer.
Color variation caused in a plane due to misregistration can be reduced up to a level at which the color variation is not a problem visually by measures including optimization of the mechanism and the control of a driving system, adoption of a rotational screen for differentiating the screen angle of each color, optimization of the number of lines and form of a dot and further, forming a mark for detecting misregistration on an image carrier, measuring misregistration by detecting this mark by a sensor and correcting an image recorded position based upon the amount of measured misregistration. However, as for color variation caused in a plane because of an image formation process such as the electrification, the exposure, the developing and the transfer, decisive possible solution is not proposed as described later. Therefore an electrophotographic image formation device wherein color variation is reduced up to a level at which the color variation is not problem visually has not been realized yet.
Characteristic color variations of color variations caused in a plane in an electrophotographic image formation process will be described below. In an electrophotographic image formation device generally utilized for a copying machine and a printer, an image is formed by exposing an image part by an optical scanner after a photo conductor is evenly charged and making charged toner electrostatically adhere to the photo conductor by a developer. Generally, the photo conductor has in-plane unevenness in the charge and optical sensitivity and has a problem that even if uniform charge, uniform exposure and uniform developing are performed, dispersion occurs in density thereof depending upon a two-dimensional location. The cause in which in-plane unevenness is caused in the charge and the optical sensitivity of the photo conductor greatly depends upon the manufacturing method and the structural problem of the photo conductor. Normally, a photo conductor is manufactured by applying sensitive material such as organic sensitive material onto a pipe or a belt made of conductive material such as aluminum. The sensitive material applied at this time is a few tens um thick and is very thin. The charge and the optical sensitivity of the photo conductor are greatly influenced by the thickness. Therefore, the thickness is required to be unified, however, as the sensitive material is required to be precisely applied in units of micron for that purpose, the cost of the photo conductor is increased and the method is not realistic.
Further, there is a problem that it is difficult to uniformly charge and expose a photo conductor and to uniformly develop on the photo conductor. In developing, unless an interval between a developing roll and a photo conductor is fixed, the quantity of toner used for developing differs and the amount of toner that adheres to the photo conductor disperses. The developing roll and the photo conductor are normally apart by a few hundreds μm. Therefore, high mechanical precision and rigidity are required for a frame for fixing the developing roll and the photo conductor and there is a problem that the cost of the image formation device is increased and the image formation device is large-sized.
In case that a laser scanner is used for exposure, a beam diameter on a photo conductor in a direction of horizontal scanning differs depending upon positional precision between the optical scanner and the photo conductor and unevenness is caused in the electric potential of the photo conductor in the direction of horizontal scanning.
In addition, in a color electrophotographic image formation device, an image is formed by transferring a toner image formed on the photo conductor onto a recording paper on an intermediate transfer belt or on a transfer belt. Generally, as the volume resistivity of the transfer belt is not uniform in a plane of the belt, there is a problem that color variation is caused depending upon a location in a two-dimensional plane. A reason why the color variation is caused by the transfer belt is that it is difficult to manufacture the transfer belt so that the thickness is uniform overall. To adjust the volume resistivity of the transfer belt, carbon black is mixed in plastic such as polyimide which is a base of the transfer belt, however, at that time, as the carbon black is not uniformly dispersed in the plastic, the volume resistivity in a plane of the transfer belt is not fixed.
The transfer efficiency differs depending upon a location of the transfer belt because of such a manufacturing problem. Therefore, when fixed transfer current flows, the density of a transferred image varies to cause color variation. Particularly, it is difficult in manufacture to unify the volume resistivity of an overall transfer belt the transfer efficiency of which is satisfactory, that is, the volume resistivity of which is small and such a transfer belt has a defect that color variation is remarkably caused depending upon a location in a two-dimensional plane of paper. To unify the volume resistivity of the transfer belt, the transfer belt is required to be manufactured so that the thickness is uniform overall and carbon black is required to be uniformly dispersed in the plastic which is the base thereof, however, this causes the increase of the manufacturing cost and is not realistic.
Such an electrophotographic image formation device has a cause that causes in-plane color variation in image formation process thereof including charge, exposure, developing and transfer and color variation is caused in an image on paper depending upon a location in the two-dimensional plane as synthesis of the color variation caused in each image formation process.
In the meantime, in JP-A-6-135051 for example, an image formation device that calculates one-dimensional density correction tables of a horizontal scanning direction and a vertical scanning direction by reading an image pattern formed on a photo conductor and corrects color variation caused in its image formation process by correcting a picture signal referring to the density correction table is disclosed. The method of correcting a picture signal referring to the one-dimensional density correction tables of the horizontal scanning direction and the vertical scanning direction as described above is effective to the correction of the color variation caused by a cause in one dimension being independent of the horizontal scanning direction and the vertical scanning direction. However, since color variation caused on an image carrier such as a photo conductor and a transfer belt exists at random in a two-dimensional plane, it is impossible in principle to correct color variation in the two-dimensional plane by only one-dimensional correction.
Also, in JP-A-5-227396 for example, an image formation device that records an image having fixed density on overall paper, reads the image to store calculated correction value, and corrects in-plane color variation by correcting a read image of a manuscript according to the correction value to output the corrected image when reading the image of the manuscript to perform image recording, and thereby corrects the color variation in a plane of paper is disclosed.
Normally, in an electrophotographic image formation device, it differs depending upon the configuration whether paper and an image carrier such as a photo conductor and a transfer belt are synchronous or not. For a typical example in which paper and an image carrier are synchronous, an image formation device that forms a toner image of each color on one photo conductor and forms a color image by sequentially transferring the toner image of each color on paper on a transfer drum maybe given. In case that the transfer drum is used, since the photo conductor and the transfer drum are synchronized to prevent misregistration, a paper, the photo conductor which is an image carrier and the transfer drum are completely synchronous. Therefore, as represented by this example, in the image formation device wherein the photo conductor and the transfer drum are synchronous, the color variation in a plane of paper has reproducibility.
In the meantime, for a typical example in which paper and an image carrier are asynchronous, an image formation device that forms a plurality of toner images of colors corresponding to a plurality of photo conductors, respectively, and forms a color image by transferring toner images on a paper together after the toner images of the colors corresponding to the plurality of photo conductors is sequentially transferred on an intermediate transfer belt may be given. In case that the intermediate transfer belt is used, since a slight slip is caused between the photo conductor and the intermediate transfer belt to absorb a mechanical dimensional error and an operational error, the photo conductor and the intermediate transfer belt are asynchronous. Normally, since the image formation device is configured so that a reference position of the intermediate transfer belt is detected by a sensor and an image is always transferred in the same position of the intermediate transfer belt, the intermediate transfer belt and a paper are synchronous. Therefore, as paper, the intermediate transfer belt and the photo conductor are not completely synchronous, the color variation in a plane of paper does not have reproducibility in this method.
Therefore, such method of calculating the correction value based upon the output of the image of the fixed density on overall paper and correcting the picture signal according to the correction value as is proposed in JP-A-5-227396 is considered to be effective to the correction of the color variation in the two-dimensional plane of the image formation device in which the paper and the image carrier are synchronous. However, as in-plane color variation varies because the paper and the image carrier are in relative positional relation in the image formation device in which the paper and the image carrier such as a photo conductor and an intermediate transfer belt are asynchronous, such in-plane color variation cannot be corrected.
Further, in the methods proposed in JP-A-6-135051 and JP-A-5-227396, the correction value is determined by measuring the density of the reference image of the fixed density. However, gradation in charge is nonlinear and the amount of the color variation in a plane differs depending upon density. Therefore, there is a problem that when the correction is made based upon the correction value determined based upon the image of an intermediate density as described in these documents, an error is caused in a part of low density and a part of high density, the color variation is not corrected and color variation by excessive correction appears as a defect of an image by irregular color.
In addition, in the methods proposed in JP-A-6-135051 and JP-A-5-227396, the correction value is determined by measuring monochromatic toner. Supposing that these methods are applied to a color image formation device, the same amount of correction is applied to each color. However, when a color image is formed in electrophotography, multiple transfer is performed. The nonlinearity of the multiple transfer has a problem that in a color image of second order color or higher order, a large error is caused by the amount of correction determined based upon a monochrome.
The nonlinearity of multiple transfer means that in case cyan (hereinafter called C), magenta (hereinafter called M) and yellow (hereinafter called Y) for example are output at the ratio of the area of a dot of 50%, C, M and Y are multiply transferred on a transfer belt in the order and gray is output, the toner of C first transferred is heavier than that in the case of a monochrome and the toner of Y finally transferred is lighter than that in the case of a monochrome respectively because of the nonlinearity of multiple transfer. As difference in the weight of toner by multiple transfer normally exists by 10 to 20% in this example, it is clear that a large error is caused in a correction according to the amount of correction determined based upon a monochrome in the case of second color or more.
As described above, in the color conversion according to the related art, since an effect by in-plane color variation cannot be completely considered in the electrophotographic image formation device the in-plane uniformity of which is low even if a high-precision color conversion system represented by DLUT is applied, high color conversion precision cannot be acquired. Also, in the color conversion according to the related art, when a uniform image is output by using an image formation device the in-plane uniformity of which is low, irregular color caused due to in-plane color variation is also perceived as a defect of an image.
In electrophotography, in order to enhance in-plane uniformity, a method of recording an image of fixed density in a plane and correcting a picture signal based upon its measured value is proposed, however, as in this technique, the nonlinearity of a gradation characteristic of electrophotography and the nonlinearity of a multiple transfer characteristic of electrophotography and synchronization between an image carrier and paper are not considered, in-plane color variation cannot be completely corrected in principle.
In the meantime, to enhance in-plane uniformity by improving an image formation process including electrification, developing and transfer, the increase of the cost of a photo conductor and a transfer belt, a large-sized apparatus and high precision are required and are not realistic.